(a) Field of the Invention
The invention relates to apparatus and methods for improving the drive of a vehicle on a steel rail by means of travelling magnetic fields whose lines of force pass transversely through the rails.
(b) Prior Art
The first linear induction motor, applied to railroad traction, in terms in which it is usual today to describe such motors, was patented by Zehden in 1905 as referred to by Chirgwin in Recherche sur les moteurs lineaires a induction aux Etats-Unis, in "Bulletin de l'A.I. du Congres des Chemins de Fer", Dec. 1967, p. 793. Zehden's Patent was forgotten and it was Professor Laithwaite, then at the University of Manchester, who resurrected, half-a-century later, the idea of the linear motor, although, in the words of Pierre de Latil, "he did not foresee, as an application, anything else than the shuttle movements on weaving-machines" (P. de Latil--Le metro sous coussin d'air, in "Sciences at Avenir", No. 234, Avril, 1968, pp. 304-305). Laithwaite also promulgated afterwards, the theory of the linear motor (E. R. Laithwaite--Induction Machines for Special Purposes, George Newnes Ltd., London 1966.) and, from his work and that of Poloujadoff, in France, and of W. W. Seifert, in the United States, among others, an explosion of interest came about recently regarding linear electric motors applied to railroad traction.
In general, these motors consist of a linear three-phase primary winding (inductor) and a fixed secondary formed by a special rail made of metal of high electrical conductivity, this rail being different in construction and separate from the rails used for supporting and guiding the train in conventional railroads.
An exception is found in British Pat. No. 1,200,201 (TASSIN). The secondary of the induction motor in this British Patent, is, as in the present invention, the supporting rail of the railroad. However, Tassin's induction motor is a longitudinal flux machine; the lines of force of its magnetic field are contained in a plane that contains also the direction of motion of the travelling field. It is the equivalent of a conventional rotary induction motor, the stator of which has been opened out flat.
The machine in the present invention, as will be shown later on, cannot be obtained by such a simple operation as opening out flat, in thought, a machine of conventional design. The lines of force of its magnetic travelling field lie in planes that are perpendicular to the direction of travel of the field: it is a transverse flux machine. This machine is clearly a new departure as regards conventional induction motors; it is different from Tassin's motor both in concept and in performance.
The machine in the present invention differs also from the motor described in U.S. Pat. No. 3,585,423 (BOLTON) in at least one feature: it belongs to a different family of transverse flux machines. This difference will become readily apparant if one makes the "thought-operation" opposite to the above mentioned one: coiling round in a ring the linear machines in this comparison. From the Tassin motor a conventional induction rotary motor (longitudinal flux) is at once obtained.
From the Bolton-Lauthwaite's motors there results transverse flux machines, the topology of which is "not convenient or profitable"; "a rotary t.f.m. is cumbersome to construct" says Prof. Laithwaite in the Electrical Times (see "Inverter System is not vital to traction by linear motor"--Electrical Times, Oct. 18, 1974, pgs. 6-7).
From the machines described in the present invention result transverse flux machines that are not cumbersome at all. In the pure induction configuration these machines are yet to be manufactured but, when they are, they cannot be mistaken with those of Prof. Laithwaite which "are cumbersome to construct". In the configurations that include a system of electrical brushes, some of the resulting machines do exist already. It is the case of the d.-c. homopolar generator which is a limiting or degenerate form of these machines. The homopolar generator is a rotary transverse flux machine that corresponds to the transverse flux shoes braking device described by Parody et Tetrel in 1935, when to this device are added a non-null air-gap and a system of brushes to collect the emfs. generated in the secondary. This device was in the origin of the conception of the machines in this invention.